The preparation of Ziegler-Natta catalysts by supporting a titanium compound and an electron donor compound on an active magnesium halide support is well known in the art.
An adduct of a magnesium halide and an alcohol is often used as the active magnesium halide support, and reacts with a titanium halide and an electron donor compound to give a spherical catalyst component, which, together with a cocatalyst and optionally an external electron donor compound, constitutes a catalyst. When used in olefin polymerization, in particular in propylene polymerization, such a catalyst exhibits a relatively high polymerization activity and a higher stereospecificity.
Known magnesium halide/alcohol adducts comprise generally binary components: magnesium dichloride and an alcohol. Some of known alcohol adducts further comprise a minor amount of water. Such alcohol adducts may be prepared by known processes, such as spray drying processes, spray cooling processes, high-pressure extruding processes, or high-speed stirring processes. The magnesium dichloride/alcohol adducts are described in, for example, U.S. Pat. No. 4,421,674, U.S. Pat. No. 4,469,648, WO 8707620, WO 9311166, U.S. Pat. No. 5,100,849, U.S. Pat. No. 6,020,279, U.S. Pat. No. 4,399,054, EP 0 395 083, U.S. Pat. No. 6,127,304 and U.S. Pat. No. 6,323,152.
It has been found that when the catalysts prepared from such a magnesium dichloride/alcohol adduct are used in olefin polymerization, a cracking phenomenon of catalyst particles as well as polymer particles takes place easily so that there are overmuch polymer fines. A main reason might be that catalytic active sites formed in the adduct supports by reacting the adducts with titanium halides and electron donor compounds are not uniformly distributed. In order to overcome this drawback, it has been attempted to introduce the electron donor compound during the preparation of the magnesium dichloride/alcohol adduct supports. For example, the technique as disclosed in Chinese Patent ZL02136543.1 and CN1563112A introduces an internal electron donor well-known in the art, such as a phthalate, in the preparation of the support so as to form a spherical “magnesium dichloride-alcohol-phthalate” multi-component support, which then reacts with titanium tetrachloride to form a catalyst component. However, because the spherical multi-component support is likely viscous during the preparation thereof, it is difficult to form spherical particles having a desired particle diameter (the disclosed spherical supports have average particle sizes, D50, in the range of from 70 to 200 microns). Furthermore, when used in propylene polymerization, the catalyst exhibits a catalytic activity of at most 406 gPP/gcat. Therefore, the catalyst is not satisfied.
Chinese patent application CN101050245A discloses a spherical adduct support having a general formula of MgX2.mROH.nE.pH2O, wherein E is a gem-dihydrocarbyloxy hydrocarbon compound. The support has a narrower particle size distribution and the average particle size of the support can be easily controlled. The catalyst component prepared by reacting the adduct support with a titanium compound has a good hydrogen response when used in olefin polymerization, in particular in propylene polymerization, and the resultant polymer has a good particle morphology. However, when the catalyst component is used in propylene polymerization at a higher hydrogen concentration, its stereospecificity, especially the isotacticity index of the obtained polymer having a higher melt index, needs to be further enhanced.
Chinese patent application CN85101441A discloses a process for the polymerization of an alpha monoolefin in the presence of certain supported coordination catalyst systems which comprise (a) a procatalyst, (b) a cocatalyst, and (c) a selectivity control agent, wherein (a) is a solid composition comprising magnesium dichloride, titanium tetrachloride, and an electron donor; (b) is an aluminium trialkyl; and (c) is a combination of a strong selectivity control agent and a weak selectivity control agent. Said strong selectivity control agent may be an alkoxy benzoate, preferably a p-alkoxy benzoate, and the weak selectivity control agent is an ether or a tertiary amine. However, in the case of propylene polymerization, the resultant polymers have lower isotacticity indices, when either the alkoxy benzoate alone or the combination of the strong selectivity control agent and the weak selectivity control agent is used.
Chinese patent application CN1743347A discloses the incorporation of o-alkoxybenzoate compounds as internal electron donor into olefin polymerization catalyst components. Said patent application discloses that, when a combination of an o-alkoxybenzoate compound and a phthalate compound is used as internal electron donor, the hydrogen response of the resultant catalyst can be improved. However, by using such a catalyst, the obtained polypropylene having a high melt index will have a lower isotacticity index.
Thus, there still need catalysts for olefin polymerization, which will exhibit a relatively high polymerization activity, a higher stereospecificity and the performance that can obtain higher isotactic index polypropylene with higher melt index at a higher hydrogen concentration, and which will give a polymer having a good particle morphology and a higher bulk density.